The FBI acknowledged yesterday that some of the terrorists involved in the attacks last week were using false identities, as it emerged that at least two men had been wrongly implicated.

After analysis of the passenger lists of the four hijacked flights and other immigration documents, investigators identified Salem Al-Hazmi and Abdulaziz Al-Omari as two of the terrorists.

The real Salem Al-Hazmi, however, is alive and indignant in Saudi Arabia, and not one of the people who perished in the American Airlines flight that crashed on the Pentagon. He works at a government-owned petroleum and chemical plant in the city of Yanbu.

He said yesterday he had not left Saudi Arabia for two years, but that his passport had been stolen by a pickpocket in Cairo three years ago.

Abdulaziz Al-Omari has also come forward to say he was not on the flight from Boston that crashed into the north tower of the World Trade Centre.

An electrical engineer who works in Saudi Arabia, Mr Al-Omari said he was a student in Denver during the mid-1990s, and that his passport and other papers were stolen in a burglary in the US five years ago.

He was given a special pass by the Saudi embassy so he could return home.

"The name is my name and the birth date is the same as mine," he told Asharq al-Aswat, a London-based Arabic newspaper. "But I am not the one who bombed the World Trade Centre in New York."

Both men have offered to fly to the US to prove their innocence.

A Saudi embassy official in Washington warned yesterday that many more terror suspects might have been using false identities.

"The Salem Al-Hamzi we have is 26 years old and has never been to the United States," Gaafar Allagany told the Washington Post. "He has said he is willing to come to the United States if anyone wants to see him."

The FBI said it was reviewing the information about those on board the flights and that "the possibility that some of the identities are in question is being actively pursued".

The confusion has added to the problems of investigators. They have discovered that one of the men arrested, Badr Mohammed Hamzi, a radiologist from San Antonio, Texas, regularly used the name Khalid Al-Midhar, who has been named as another of the hijackers.